Conventionally, there is provided a discharge lamp electronic ballast configured to convert DC power (direct-current power) from a DC power supply into AC power (alternating-current power) to power an HID lamp (a high intensity discharge lamp) or the like, i.e., supply the AC power thereto.
HID lamps such as metal-halide lamps with high luminous flux are used for vehicles. In mainly used conventional lamps, mercury is enclosed in such a lamp in order to start the lamp to increase its luminous flux and stabilize the lamp so that a voltage across electrodes of the lamp is set to be rather high. A lamp in which mercury is enclosed is called a D1 or D2 lamp in general, and the D1 lamp has an igniter which is built in the lamp and configured to generate ignition trigger pulses, whereas there is a mercury-free lamp made to replace mercury with other halogen compound from the point of view of an environmental problem, and the market is now expected to expand. The mercury-free lamp is called a D3 or D4 lamp, and the D3 lamp has an igniter which is built in the lamp and configured to generate ignition trigger pulses.
For example, Japanese Patent Application Publication No. 2002-216989 A discloses discharge lamp electronic ballast configured to output a power command larger than a maximum power limit for several second from a point in time when a discharge lamp is lit, and to output a power command corresponding a rated output after several tens of seconds. In this ballast, the maximum power limit is adjusted in response to a temperature detection value from a temperature detector, thereby suppressing the increase of an internal temperature of the ballast.
In an HID lamp for vehicle, there is a problem of increase of the electricity capacity due to a ballast, wiring or the like, and generation of heat, because if the mercury evaporates, a lamp voltage decreases (e.g., from 85V to 42V) and a lamp current needs to be increased in general. In addition, if the discharge lamp electronic ballast is miniaturized, the temperature of the ballast increases, and accordingly the output to the lamp needs to be decreased, but an excessive decrease of the output may cause lamp flicker, and lamp going out during operation (hereinafter referred to as a “lamp-out”).